1. Field
The present invention relates to an inverter device which can drive two-phase induction motors with high efficiency.
2. Related Art
Electric motors used in industrial instruments and household electrical appliances need to be driven with high efficiency. Eminent techniques for this purpose include driving a brushless DC motor such as a permanent magnet motor by an inverter device. This motor-driving technique is employed in air conditioners, refrigerators and the like. However, the permanent magnet motors are expensive so that disseminating the permanent magnet motors to the world has a certain limit. On the other hand, a single-phase induction motor includes a main winding directly connected to a single-phase power supply and an auxiliary winding connected via a phase advance capacitor to the single-phase power supply. Since the single-phase induction motors have lower production costs, these motors have a wide distribution in the world. In view of importance of recent energy saving, it has been a technical problem how the single-phase induction motors could be driven with high efficiency.
As one of techniques for driving a single-phase induction motor by an inverter, Japanese patent application publication JP-A-H02-111288 discloses a motor driving arrangement including three-arm semiconductor switching elements connected to a main winding terminal, an auxiliary winding terminal and a common terminal respectively. Three sinusoidal voltages phase-shifted by 90 degrees are delivered as PWM signals so that 90-degree phase shifted sinusoidal voltages are supplied to the main and auxiliary windings. Furthermore, Japanese patent application publication JP-A-S64-8896 discloses a technique for vector-controlling a three-phase induction motor by a position sensorless control method.
However, the induction motor cannot be driven with high efficiency even when the 90-degree phase shifted sinusoidal voltages are supplied to the main and auxiliary windings, as disclosed by JP-A-H02-111288. In order that a single-phase induction motor may be driven with high efficiency, current with an optimum amplitude needs to be supplied to the main and auxiliary windings with 90-degree phase shift. Furthermore, although JP-A-S64-8896 discloses a sensorless vector control for a balanced three-phase induction motor, the disclosed control manner cannot in situ be applied to a two-phase induction motor comprising main and auxiliary windings having different winding specifications.